Comment by PhilMH:
Sorry WM, I don't know for sure, but I note that Michael de Looper's London listing has this as March 1957, so Michael might like to chime in here if he's lurking? The matrix numbers XLON1 and XLON2 might be a clue - I note that in 45cat, the 7XLON matrices seem to have started appearing on labels in early 1957, and I found some fairly low 2-digit numbers there, so presuming that LPs and singles used the same numerical series, then it looks like this series started in 1957.
Comment by wax_museum:
Discogs has this at 1957, but I think it’s more like 1955 because the London release of Green Tree Boogie on 45 was ‘55 and the UK releases on London were also around this date. Phil? Any ideas on this?